With the significant rise in the use of the optical communications, the number of channels within a fixed frequency range has also been rising. One of many solutions implemented to accommodate growing number of channels is the reduction of the spacing between two adjacent channels. For example, in the last few years 50 GHz channel spacing was common; now, the channel spacing of 37.5 GHz or less is desirable. Decreased channel spacing allows for increased spectral efficiency and the new optical channels within the saved spectrum. One of the disadvantages of decreasing the channel spacing, however, includes a reduced ratio of the channel spacing to the channel baud rate. This reduced ratio leaves a reduced margin for misalignment errors (particularly, with respect to laser central frequency errors).
The lasers in the optical communication system typically exhibit drift in their frequency, for example around ±1.8 GHz. This drift in the laser frequency is often sufficient to produce a large bit-error rate (BER) penalty in the optical communication system. Known coherent optical transmitters/receivers do not have an internal mechanism to monitor drift in the channel spacing. The known monitoring methods use an externally-connected instrument(s) such as optical spectrometer.
Accordingly, a need exists for a method to monitor digitally and correct the spacing between the adjacent channels in the coherent optical transmission systems.